1. Field of the Invention
The present invention relates generally to a lens barrel of a camera, and more particularly to a lens barrel of a camera, in which a cam groove is formed in an internal circumferential surface of a rotatable cylinder of the lens barrel, and a cam follower of a lens holding frame engages with the cam groove so that the lens holding frame can move forward and backward within the rotatable cylinder upon rotating of the rotatable cylinder.
2. Description of Related Art
In order to change a focal length, a conventional zoom lens of a camera moves two or more lens groups relatively to one another by a cam mechanism arranged within a rotatable cylinder supporting the lens group therein. The conventional cam mechanism is constructed in such a way that a cam groove is formed in the internal circumferential surface of the rotatable cylinder, and a cam follower provided on a lens holding frame, which holds a lens, engages with the cam groove. The rotation of the rotatable cylinder causes the cam follower and the lens holding frame to move forward and backward along the cam groove within the rotatable cylinder. The rotatable cylinder is fitted in another cylinder that is fixed or movable. A helicoidal thread formed on the external circumferential surface of the rotatable cylinder engages with a helicoidal thread formed on the internal circumferential surface of the other cylinder, so that the rotatable cylinder moves forward and backward with respect to the other cylinder upon being rotated.
Conventionally, the looseness between the cam follower and the cam groove causes the cam follower to come in contact with one of the front edge and the rear edge of the cam groove dependently upon the rotational direction of the rotatable cylinder. Hence, even if the rotatable cylinder is located at the same position of a rotational angle, the position of the lens holding frame varies dependently upon the past rotational direction of the rotatable cylinder. As a means to solve this problem, there is a well-known pressing mechanism, which always presses the lens holding frame forward or backward with respect to the rotatable cylinder in order to always press the cam follower against the front or rear edge of the cam groove independent of the rotational direction of the rotatable cylinder. For example, the tension spring presses the lens holding frame backward with respect to the rotatable cylinder so that the cam follower can always be in contact with the back edge of the cam groove.
However, this pressing mechanism using the tension spring has a disadvantage in that the drawing force of the spring is different between a wide side and a telephoto side of the zoom lens, thus causing a great change in the rotational load during the rotation of the rotatable cylinder.